Neural interfaces enable a connection between nervous tissue and the ex vivo environment. These devices are not only useful for neuroscience research, but also provide therapy for patients afflicted with a multitude of neuronal disorders. The advent of optogenetics, a new technique involving genetic modification of neural cells to make them susceptible to light stimulation, has not only revolutionized neuroscience research, but also transformed the requirements for neural interfacing devices. It is now desired to optogenetically stimulate the cortex with light while simultaneously recording the evoked response. Neural surface electrode arrays, such as micro-electrocorticography (micro-ECoG) devices, strike a balance between invasiveness and recorded signal quality. However, these devices use opaque metallic conductive materials. Thus, it is possible to stimulate around the electrode sites, but not directly at the electrode-tissue interface. Additional advancements in in vivo imaging modalities could provide valuable information regarding the tissue response to implanted electrode arrays, and help correlate tissue behavior with recorded signals. To date, however, these methods have mainly been used to image tissue surrounding micro-ECoG electrode sites, since imaging at the electrode-tissue interface is infeasible, due to the conductor opacity.
Transparent micro-ECoG arrays have been fabricated using indium-tin oxide (ITO), a transparent conductor commonly used in solar cells. ITO, however, is not ideal for employment with micro-ECoG technology, for a variety of reasons. ITO is brittle and requires high-temperature processing not suitable for use with the low-glass-transition-temperature polymer substrates. In addition, ITO has process-dependent transparency, which is rather limited in the ultraviolet (UV) and infrared (IR) wavelength ranges that are used for stimulating various opsin types and visualizing fluorescently tagged cells in neural imaging and optogenetic applications.